Changes in whistle structure of resident bottlenose dolphins in relation to underwater noise and boat traffic.

The habitat of the resident bottlenose dolphins (Tursiops truncatus) of the Cres-Losinj archipelago overlaps with routes of intense boat traffic. Within these waters, Sea Ambient Noise (SAN) was sampled across ten acoustic stations between 2007 and 2009. Data on boat presence was concurrently collected and when dolphins were sighted group behaviour was also recorded. Acoustic recordings were analysed for 1/3 octave bands. Samples containing dolphin whistles were analysed and compared with boat presence and SAN levels. Results indicate that dolphins whistle at higher frequencies in conditions of elevated low frequency noise. Conversely, they reduce maximum, delta and start frequencies and frequency modulations when noise levels increase significantly across higher frequencies. The study shows that high levels of SAN causes significant changes in the acoustic structure of dolphin whistles. Additionally, changes in whistle parameters, in the presence of boats, appear to be related to the behavioural state of the dolphin group.

Are the 1/3-Octave Band 63- and 125-Hz Noise Levels Predictive of Vessel Activity? The Case in the Cres-Losinj Archipelago (Northern Adriatic Sea, Croatia).

A 3-years sea ambient-noise (SAN) monitoring was carried out in the Cres-Losinj Archipelago (Croatia), where a bottlenose dolphin population is threatened by unregulated nautical tourism. A total of 540 5-min SAN samples were collected and analyzed in an Indicator 11.2.1 (Marine Strategy Framework Directive) perspective. The 1/3-octave band center frequencies of 63 and 125 Hz (re 1 µPa.rms) proved to be predictive of local predominant ship type over time. However, the noisiest band level was centered on 200 Hz. We therefore suggest measuring a wider frequency band than those requested in Indicator 11.2.1.